As the safety lifecycle (SLC) methods of ISA84.01 and IEC61508
are becoming more widespread, engineers who design safety
instrumented systems (SISs) are starting to think more about
the valves they use. Remember that the standards use a
quantitative approach to safety verification for the hardware.
A given set of equipment intended to protect against a specific
hazard (a safety instrumented function, SIF) has a requirement
to achieve a certain risk-reduction factor. This is established
during risk analysis and stated in the safety requirements
specification. Failure rates and modes of the various pieces of
equipment are input to a reliability and safety analysis
model that results in calculating the achieved risk reduction
factor, RRF, probability of failure on demand average, PFDavg,
safe failure fraction, SFF, and other metrics. The results
achieved must meet or exceed the requirements.